In the beta version of activity diagrams, the combination of a backward reference and swim lanes does not work.
This is the combination that produces a broken diagram (the backward node flows somewhere in the middle)
There is another same question from 2020 in here https://forum.plantuml.net/12427/backward-in-activity-beta-does-not-work-with-swim-lanes
repeat :Generate Again;
backward: Do bar;
repeat while (Is foo?) is (yes)